Whenever I go from a sitting to standing position, OR if I even strain the slightest bit on the toilet (not really straining, just normal push for movement) or if I hold my breath for a few seconds, WHY does my head throb???? It feels like there's an enormous rush of blood to my head, I have a fleeting mild headache that goes away within a few seconds, and I can feel my pulse VERY hard in my face, head, neck, arms, ect. This is one of the main reasons I think I have a brain tumor.

Those sound like orthostatic changes in your blood pressure and when on the toilet it's probably vagal nerve stimulus. Both can give yyou the rush / full feeling in your head. Talk to your GP and have a tilt table test.

I agree with people who are saying it's vagus nerve stimulation and changes in blood pressure. When I get anxious, I sometimes feel/see a throbbing on the sides of my head, my Doctor said it wasn't anything to lose sleep over and to stop worrying so much.